Conventional rotation shafts provided in various drive force transmission mechanisms are rotatably supported via bearings. For such rotation shafts, so-called ball bearings are used in order to improve rotation performance of the rotation shafts. Such ball bearings include multiple rolling members (rolling elements) circumferentially disposed between inner and outer rings.
Such bearings are used as support members for rotation shafts of drive force transmission mechanisms in various drive units, and some drive units are desired to prevent foreign substances such as dusts and water from penetrating through the bearings into the units. Moreover, when foreign substances enter into the bearings themselves, rotation performance of such shafts could be deteriorated and noise could be caused. For this reason, Japanese Patent Application Publication No. S57-033222 (the “'222 Publication”) and Japanese Patent Application Publication No. 2013-228044 (the “'044 Publication”) for example disclose a magnetic fluid sealed bearing having a sealing ability using a magnetic fluid.
In the magnetic fluid sealed bearing disclosed in the '222 Publication, an annular magnet is disposed between inner and outer rings on one side of the opening of the bearing to form a magnetic circuit extending from the outer ring through rolling members to the inner ring, and a magnetic fluid is retained between the magnet and the end surface of the inner ring facing the magnet to prevent foreign substances from entering inside. In the magnetic fluid sealed bearing disclosed in the '044 Publication, a ring-shaped magnet having a ring-shaped polar plate is disposed on opening portions of the inner and outer rings to form a magnetic circuit on both the inner ring side and the outer ring side and retain a magnetic fluid in both a gap on the inner circumferential surface of the outer ring and a gap on the outer circumferential surface of the inner ring, so as to prevent foreign substances from entering from the outer ring side and the inner ring side.
As in the above known techniques, it is necessary to dispose a magnet between inner and outer rings to retain a magnetic fluid in gaps between the inner and outer rings. However, because poor dimension accuracy of the magnet causes irregularity of magnetism in the portion to be magnetically sealed, the resultant sealing performance is instable. Particularly because a ring-shaped magnet is applied onto a step in the inner and outer rings, poor dimension accuracy of the magnet makes it difficult to build in the magnet into the bearing while retaining the concentricity of the magnet and the polar plate, resulting in low work efficiency. In addition, if the magnet is positioned by itself, the sealing performance cannot be stabilized because the magnet may be damaged by an impact during the build-in work or an external impact.